Two of the most common methods of applying images to a substrate are by direct painting or screening, or by the use of die-cut, weeded, and premasked film. The former approach is time consuming and expensive, requiring relatively skilled labor, long application times, and can potentially contaminate adjacent areas. As for the latter, die-cutting and weeding represent substantial expenditures of time and money, and do not lend themselves to the manufacture and transfer of small images such as fine lines, halftones, etc. Such a system typically involves a polymeric film bearing a graphic design with a layer of adhesive under the graphic design protected by a liner. To provide the desired design, such films are die and/or "kiss cut". The resultant design is then bonded to a desired substrate via the adhesive layer after liner removal.
To overcome these noted deficiencies, much attention has been directed in the literature to the development of self-weeding, dry transfer assemblies.
Included in these approaches are those wherein adhesive is printed onto and only onto the graphic design, such as disclosed in U.S. Pat. Nos. 4,028,474 (Martin), 4,028,165 (Rosenfeld), and 4,421,816 (Arnold). This approach is difficult in that registration must be exact and as such is especially critical for the transfer of fine lines, halftone dots, etc.
Another approach includes that wherein the adhesive is applied over the entire surface of the support sheet and graphic design, relying on various mechanisms for adhesive cleavage at the edges of the image areas.
Specifically, U.S. Pat. No. 3,987,225 (Reed et al.) and British Patent No. 959,670 (Mackenzie) disclose articles wherein adhesive shear is induced at the edges of the indicia to assist in differential transfer, by incorporation of a solvent or dispersing powder in the adhesive.
Others have used the concept of migrating components to advantage. In U.S. Pat. No. 4,177,309 (Shadbolt) a polar wax serves to lower the tack of adhesive not in contact with image areas while simultaneously unaffecting or minimally affecting adhesive in contact with image areas. U.S. Pat. No. 3,741,787 (Tordjman) discloses solvent migration as a means of disrupting the bond between the graphic indicia and the carrier, thereby allowing for the transfer of the indicia from the carrier to the substrate. Similarly, U.S. Pat. No. 3,684,544 (Piron) discloses the utility of material displacement from the ink into the adhesive to cause differential tackification of the image-contacting adhesive relative to the exposed adhesive.
Photosensitive peel-apart or wet-development articles have been reported that are especially useful for the development and preparation of small graphics. These articles require either irradiative or thermal imaging procedures. U.S. Pat. No. 4,454,179 (Bennett et al.) discloses the preparation of a dry transfer article wherein differential tack and transfer characteristics are achieved by photochemical means. Specifically, graphics are printed onto the surface of a low energy carrier such as polypropylene, and subsequent to drying and corona treatment, both graphics and carrier are overcoated with an actinic radiation-responsive adhesive. To facilitate differential tackification, the adhesive is exposed using the graphics as the exposure mask. The end result is a diminution or elimination of tackification of the exposed adhesive. The article upon use is burnished onto a substrate; subsequent removal of the carrier leaves the graphic design on the substrate while removing the adhesive not underlying the design due to this differential tackification.
U.S. Pat. Nos. 3,013,917 (Karlan et al.) and 4,111,734 (Rosenfeld) disclose dry transfer articles employing non-differentially tackified adhesives. The articles disclosed therein are made by printing ink on a low energy carrier to form a desired graphic pattern and overcoating the bottom side of the graphic pattern and the exposed portions of the carrier with an adhesive. Application to a substrate is provided by contacting the article to the substrate, applying pressure, and removing the carrier which desirably also removes the weed, i.e., non-image adhesive. When used with high dry tack adhesives, transfer articles comprising low energy carriers such as disclosed by Karlan and Rosenfeld typically do not reliably provide good weeding characteristics, i.e., the adhesive is not completely removed from the substrate. Therefore, such articles and typically require the use of low dry tack adhesives which in turn require high pressure or point pressure, i.e., 50 pounds/inch.sup.2 or more, to achieve graphic transfer to the substrate. Also, these references teach that the graphic pattern is formed by applying ink to the carrier such that the ink wets out the carrier. Thus, the ink is printed in indirect fashion. Furthermore, such graphic patterns may tend to be difficult to separate from the carrier, thereby resulting in incomplete transfer to the substrate and/or spoiling of the finish of the transferred graphic pattern.